test: Add tests for NativeAllocator.ApplyProtection()

[myarichuk]

🎯 What: The testing gap addressed
This PR addresses the lack of test coverage for NativeAllocator.ApplyProtection().

📊 Coverage: What scenarios are now tested

• Edge cases: passing null or size 0.
• Valid use cases: Applying MemoryProtectionMode.None and MemoryProtectionMode.ReadOnly dynamically.
• Sub-span protection: Modifying memory protection on a specific page of an allocation without affecting adjacent pages.
• Access Violation constraints: Verifying that writing to MemoryProtectionMode.ReadOnly memory throws an unmanaged crash (which is handled safely by isolating the crash in a child process, making the test portable and stable across CI pipelines).

✨ Result: The improvement in test coverage
The test coverage for NativeAllocator is now much more robust, ensuring the memory protection functionality behaves predictably and OS-specific functions (mprotect/VirtualProtect) correctly restrict read and write access across all supported backends.

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PR created automatically by Jules for task 604321551949943287 started by @myarichuk

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